The invention relates to improvements in drying mechanisms and more particularly to an improved very large rotary drying drum for drying products carried by hot gases through the drum.
The invention relates to hot gas drying wherein pieces of material to be dried such as wood chips, hog fuel and bagasse or strands pass through huge drums by hot drying gases wherein the moisture is evaporated from the material while it is carried through the drum. The drying drums are provided for many types of industries and the type of material to be dried and the amount of drying to be accomplished will dictate the drum size.
One type of dryer of this type is used in the pulp and paper and wood product industry as a rotary wafer dryer. These dryers are used to dry large pieces of material such as wood chips, hog fuel, bagasse or strand wafers which are dried before further processing. The dryers when used for wood chips handle materials which have a wide range in size, and the drum may be provided to dehydrate any form of material. The drum of these dryers is large, up to 18' in diameter or more and 20' to 80' in length. They are supported to rotate about their horizontal longitudinal axis and are supported on suitable rotatable supports and motor driven in rotation. The interior of these drums is provided with flights which help to distribute the material and prevent its too rapid passage axially through the drum.
One difficulty encountered with these large drums is that material tends to be carried too rapidly on the stream of gases through the drum. Also, the particles tend to agglomerate so that insufficient area exposure to the hot drying gases is obtained. Efficient heat exchange is promoted by creating a homogeneous shower or curtain of the material, to fill the entire cross section of the drum. Gas dispersion is promoted if the material is showered properly and the individual pieces of the material are liberated to fall a predetermined distance. Previous designs have been deficient in promoting these efficiencies, particularly with large materials of ten or twelve inches in length.
Dryers of this type frequently have multiple uses and must be adaptable to handling various sizes and amounts of material. Production in a given plant may vary as well. Accordingly, often different requirements are necessary for adequate dispersion of the material in the drum and particularly for adequate deterrence of the speed of travel. The particles of material being dried cannot be allowed to be swept on through the drum by the traveling gases or there will be insufficient particle drying occurring. If inadequate showering of material occurs hot streaks of gases penetrate the length of the dryer, which can result in damage to the material being dried.
The primary procedure to increase exposure of the materials to the gases is to rotate the drum so that the material is tumbled inside. Another mechanical expedient for increasing exposure of the particles to the gas is to place fins or other travel deterring projections into the gas stream so that the particles will impact the mechanical deterrents to impede the conveyance of the material by the gas stream. These, however, will not normally suffice for all types of material and the operator may not know the effectiveness of the deterrents until operation of the dryer is tried. The type of deterrent is not obvious particularly in view of the huge size of some of the dryer drums which must be provided for commercial drying purposes. The fins or projections are welded to the inner surface of the drum and to a central shaft extending axially through the drum. Once installed, the fins are not adjusted easily.
It is accordingly an object of the present invention to provide an improved dryer drum structure wherein particles of various sizes and shapes including pulverulent particles are dried by being carried by hot gases horizontally through a rotating dryer drum.
A further object of the invention is to provide improved deterrent means within the drum which will break up the materials and which will deter the rapid traveling of particles through the drum and insure improved exposure to hot drying gases.
A still further object of the invention is to provide an improved mechanical deterring means for particles carried by hot gases through a drying drum with an improved mechanical arrangement and improved adjustments to enhance the deterring effect and the dispersing effect.